A prior art flush valve device is disclosed, for example, in JAPANESE PATENT LAID-OPEN GAZETTE No. 11-324061. FIG. 32 illustrates a rear portion of a flush toilet with a prior art flush valve device mounted thereon. As shown in FIG. 32, a flush valve device 310 connected to a city water source is located in the rear portion of a flush toilet 300. The flush valve device 310 includes a flush valve body 312 and an operation unit 320 that is used to open and close the flush valve body 312. The flush valve body 312 has a main valve to open and close a main water conduit and a pilot valve to lower the water pressure in a back pressure chamber and thereby open the main valve. The operation unit 320 functions to incline a driving rod of the pilot valve, so that a supply of washing water is flown into the flush toilet 300. The operation unit 320 has a rotating handle 322 and a transmission member 324 that is formed as an axis and rotates along its axial center with a rotation of the rotating handle 324 to transmit an operating force. In response to a rotation of the rotating handle 324, the flush valve body 312 is opened via the transmission member 324 to make a supply of washing water flown into the flush toilet 300.
In order to transmit the rotational force to the flush valve body 312, the rotating handle 322 should be located on a lateral side of the flush valve body 312. This undesirably lowers the degree of freedom in positioning of the flush valve body 312 and the rotating handle 322. The restricted positions of the rotating handle 304 and the flush valve body 312 may lower the operability of the flush valve device 310 and make installation of the flush valve device 310 rather troublesome.